This proposal presents a comprehensive research program directed at gaining an improved understanding of the detailed thermal interaction between blood flow through the microvasculature and the surrounding tissue, and developing a new simplified bioheat equation for tissue heat transfer. Thermal interaction between tissue and blood has important application in the treatment and diagnosis of growing tumors, cryopreservation and cryosurgery, ultrasound and microwave radiation utilization in physical and cancer therapy, thermal sensors and blood perfusion measurements. To achieve the aims and objectives of the proposed research carefully coordinated experimental and theoretical studies are planned. These studies draw heavily on the findings and results of the current research program in which new experimental protocols have been developed and a new detailed three-layer quantitative model to peripheral tissue heat transfer was formulated and solved for representative vascular geometries. The research project is organized into five tasks which will be carried out by a research team from The City College of The City University of New York and the College of Physicians and Surgeons of Columbia Unviersiity. These Tasks are: 1. Application and extension of the three layer microvascular model to examine the effects of vascular geometry, capillary perfusion, vasodilation, cutaneous circulation, metabolic and thermalregulatory feedback control on the detailed temperature distribution in peripheral tissue. 2. In vivo experiments to measure the thermal response of thermally significant microvessels to pharmocological agents and externally monitored vessel supply conditions and to interpret the results using the above three layer microvascular model. 3. Experimental verification of the new hypothesis that counter-current microvascular heat exchange and not capillary perfusion is the primary mechanism by which blood flow alters the effective conductivity of living tissue. 4. Development of a new simplified bioheat equation based on the counter-current microvascular heat exchange mechanism which can be used to predict local blood flow using existing intratissue and surface based thermal probes. 5. To test the validity of the new bioheat equation by relating tissue thermal clearance measurements in the isolated musculature of the cat or rabbit to vascular casts that define the local microvascular structure in the vicinity of the measurement sites.